This invention relates to fasteners such as nuts, bolts, screws, washers and the like, in the field of self-locking threaded fasteners provided with teeth to resist loosening after being brought into pressure engagement with a coacting surface on a workpiece by rotating the fastener or, in the case of a washer, an element bearing against it, in a tightening direction.
Conventional toothed self-locking fasteners derive their locking action from teeth formed integrally in one or both ends. The teeth are generally saw-tooth in cross sectional shape and may extend completely across the bearing surface with only the teeth edges having more or less line contacts with the coacting surface of the workpiece and pressing progressively deeper into that surface as the fastener is tightened. These teeth edges have abruptly sloping side surfaces in the direction of rotation for loosening, and gradually sloping surfaces on the other side. This enables tightening with reasonable torques and prevents loosening. Such arrangements of saw-tooth-shaped teeth are shown in U.S. MacLean et al Pat. No. 3,078,899, Pummill U.S. Pat. No. 2,380,994, and Hagist U.S. Pat. No. 1,833,462. Attempts to control the depth of penetration of these conventional saw-tooth cross-sectioned teeth have been made by providing a continuous annular bearing ring either inside or outside the teeth. An example is shown in Junker U.S. Pat. No. 3,605,845.
Such annular bearing rings are recessed in an axial direction somewhat behind the crests of the saw-tooth-shaped teeth with the idea that when the teeth are indented a predetermined amount into the coacting surface, further indentation or penetration will be prevented by engagement with the continuous annular bearing. In practice, however, they do not work that way. The continuous annular bearing ring is either recessed too much, or not enough, with respect to the tips of the saw-tooth-shaped teeth, and considering the variations in clamping force in the bolt by manually applied torques, the teeth either do not indent sufficiently to bring the bearing ring in contact with the coacting surface, or the bearing ring engages before the proper clamping force is applied. In either case, the threaded joint can work loose.
To keep such fasteners from jarring loose under vibrating services, experts in this field have long considered it desirable that the torque required to loosen (sometimes hereinafter called the "off torque") should be at least 100% of the torque required to tighten (hereinafter sometimes called the "on torque"). Some of these conventional fasteners provide an off torque/on torque ratio substantially in excess of 100%. Such very high ratios are sometimes necessary where the design of the fastener provides wide ranges between individual fasteners, to be sure that those at the low end are at least 100%. High ratios are also preferred and specified by some designers who believe they provide greater safety margins.
The torque applied to a threaded joint is a convenient, though approximate, indicator of the bolt stress which determines the clamp load. It is most important that a desired clamp load, once applied, be maintained. One factor heretofore overlooked or inadequately dealt with in the concern for providing a high off torque/on torque ratio to keep the nut or bolt from unscrewing, is that clamp load can be lost without any back rotation of either part. Applicant has found that when a threaded joint, with a conventional toothed fastener, is subjected to vibration, or cyclic loading, the stress concentration under the tooth crests will progressively indent the relatively softer coacting surface of the workpiece and relax the bolt tension. If this continues long enough, tension will drop to the point where the bolt or nut, or both, will back off and the joint will come apart.
Thus, it is just as important, in keeping a threaded joint tight, to prevent excessive stress concentration under the teeth and thereby avoid uncontrolled indentation, as it is to provide a suitable relationship between the off torque and the on torque.